Most of us think of ice as ephemeral. It forms when it’s cold and melts when it’s warm (or pour salt on it). Yet glaciers are proof that ice can hold. The ice that forms glaciers takes centuries to form and can last for hundreds of thousands of years … and possibly many, many more.
But that’s not all! Ice is not only sustainable, it is excellent record holder. It can store information about all kinds of things in the Earth’s atmosphere such as particles (such as volcanic ash or wind-blown dust), aerosols (such as sulphurous acid) or gases (such as carbon dioxide trapped as bubbles). All this data can then be used to record changes in Earth’s climate over the long term.
Glacial ice in the Gray Glacier of southern Patagonia. Credit: Felipe Alarcon, Wikimedia Commons.
Why so? To understand ice’s book-keeping abilities, we need to look at exactly how glacial ice forms. It’s not like ice on your driveway, but rather something that forms over decades to centuries. All it takes is cold and time.
Glaciers and glacial ice begin with snow. In places that get a lot of snow each year, some of that snow may not melt during the warmer summer months. Next year’s snow then falls on top of that old snow … and the process repeats itself year after year. The oldest snow heats and cools, perhaps losing some of its mass by some melting (or sublimation – going straight to gas), but the snow will condense into ice granules called firn and then ice and finally glacial ice.
Glacial ice is as close to “rock” as water can get on Earth, forming an interlocking network of ice crystals that can look like granite under a microscope. This glacial ice may have started as many feet snow but reduced to less than an inch of ice. In this layer remain all kinds of solid particles such as ash, rock debris that fell on the ice, aerosol particles and whatever else. On top of that, air bubbles that sample the atmosphere when snow falls are trapped in the ice. You end up with layer upon layer of ice that records the annual snowfall on that glacier.
Ice crystals in Antarctic sea ice under a polarizing microscope. Credit: Sep Kipfstuhl, Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany
Glacier ice is weird. She is solid, but it can flow when tilted and under tension. Glaciers and ice sheets can be hundreds of feet to miles thick, and all that pressure can deform the ice. Ice can flow when the ice at the bottom of a glacier moves down. Here’s how a glacier advance, when snow collects on top of the glacier and the accumulation causes the bottom to flow. Conversely, as the ice melts toward the bottom of the glacier, it may retreat.
The Ice Core record
So when scientists go to places like Greenland or Antarctica, they can cut an ice core to read past conditions on Earth. The ice near the top is more recent, and the deeper you go, the older it is. Some of the oldest continuous ice cores on Earth contain annual records that go back 800,000 years.
Now there are a number of caveats when it comes to reading the ice core climate record. If the ice has been highly deformed, the lower ice has been replaced by younger ice, or the bottom of the ice has melted, then your record is much more difficult or impossible to interpret. You can also lose ice from the top through melting or sublimation, leaving older ice exposed on the surface. The question then becomes, how can you determine the age of the ice beneath your feet?
Finding really old ice
One answer may involve looking at rare elements that form when quartz is hit by cosmic rays from space. A recent study in The Cryosphere by Marie Bergelin and colleagues tries to use the sediment locked in ice cores to determine the last time that sediment was on the surface, therefore how old the ice it’s in might be. They may have found ice in the Ong Valley of Antarctica which is more than 4 million years old.
Ice core sample in Ong Valley, Antarctica. There was a lot of variation in the ice cores the team recovered, some sections were pure clear ice while others were filled with rocks and sediment. Credit: Jaako Putkonen, United States Antarctic Program
Cosmogenic nuclides are isotopes of elements such as beryllium, neon, and aluminum. They form when cosmic rays from space strike minerals, causing the elements to break down and form these rare isotopes. Cosmic rays don’t penetrate deep into rock or ice, so by looking at the amounts of those elements in the surface of the rocks and how those elements have decayed, you can model the exposure and age of burial for the material.
The Ong Valley of Antarctica is a glacial valley deep in the Transantarctic Mountains. There are some active glaciers, but also areas of old, stagnant ice that is covered by rocks and debris. Bergelin and her colleagues cut ice cores from this old ice to look at the sediment in the rocks to determine when they were buried, thus giving a minimum age of the ice.
Interpretation of the ages
It might be messy now. The ice in these valleys may not have melted (the average temperature in the Ong Valley is -11F (-24C). Instead, ice sublimates in the sun, so the young ice is lost over the years. Ong Valley can have over 70 feet (22 meters) of ice per million years. That’s a lot of ice to be, well, evaporated.
In the ~30-foot (9.4-meter) ice core that was cut by Bergelin and her colleagues, they identified multiple batches of ice and surfaces. Using measurements of cosmogenic nuclides, they found the exposure and burial age of various surfaces. Some of the youngest ice sheets were ~1.3 million years older. Deeper surfaces in the ice indicate that they were buried in the ice ~3 million years ago, and even deeper, ~4.3-5.1 million years ago.
To put this into some perspective, if these ice ages are correct, then the oldest ice formed from snows falling when Australopithecines roamed East Africa. The middle layers were formed near the time when stone tools were first used by our deep ancestors and the surface is snow from the top of Homo erectus.
The key to whether these new epochs can be used to understand climate in the past is complex. Measurements of cosmogenic nuclides can be fraught with danger based on how the samples are taken and what assumptions are used. For these ages to be accepted, the scientific community will look closely at what the authors did to determine these dates.
If they are accurate, then looking at the materials trapped in the ice of the Ong Valley could give us insight into Earth’s climate that goes back much further than we have been able to with the ice. It could also give us new tools to find even older ice elsewhere.